Electrically programmable read only memory (EPROM) must maintain the integrity of stored data for ten (10) years. In a memory cell of a prior art EPROM hot electrons are generated by impact ionization in a channel. Then some of the channel hot electrons are injected through a tunnel oxide to a polysilicon floating gate. Electrons (in amounts that represent either a zero (“0”) representation or a one (“1”) representation) are injected to the floating gate. The electron concentration in the floating gate determines the state of the data (i.e., “zero” or “one”) that is stored in the memory cell.
The thickness of the tunnel oxide between the floating gate and the substrate in a prior art EPROM is typically one hundred twenty Angstroms (120 Å). An Angstrom is 10−10 meter. A value of thickness of the tunnel oxide of 120 Å is sufficient to reduce the back-tunnel current to acceptable levels and maintain a sufficient number of electrons on the floating gate to meet the ten (10) year data integrity requirement at a temperature of one hundred twenty five degrees Celsius (125° C.).
Integrated circuit design continues to trend toward smaller and smaller dimensions. EPROM designs have recently been proposed in which the thickness of the tunnel oxide is sixty Angstroms (60 Å). This presents difficulties because the thinner thickness of sixty Angstroms (60 Å) will not provide as much isolation for the electrons on the floating gate. That is, the electrons that are stored on the floating gate will discharge to the substrate at a much faster rate when the tunnel oxide is thinner. Experience in the EPROM industry has shown that it is very difficult to obtain a ten (10) year data retention rate when the thickness of the tunnel oxide is reduced to a thickness of sixty Angstroms (60 Å). Complex and expensive measures are required to ensure that a ten (10) data retention rate can be maintained in an EPROM that has a tunnel oxide that is only sixty Angstroms (60 Å) thick.
Therefore, there is a need in the art for a system and method that is capable of providing an EPROM that has a tunnel oxide layer that is sixty Angstroms (60 Å) thick and that is also capable of meeting the ten (10) year data retention requirement.
One advantageous embodiment of the present invention provides an electrically programmable read only memory (EPROM) memory cell that comprises (1) an n-channel metal oxide semiconductor (NMOS) select transistor with a gate oxide layer that is sixty Angstroms (60 Å) thick, and (2) a p-channel metal oxide semiconductor (PMOS) breakdown transistor with a gate oxide layer that is twenty Angstroms (20 Å) thick, and (3) an p-channel metal oxide semiconductor (PMOS) program transistor with a gate oxide layer that is sixty Angstroms (60 Å) thick. The source of the NMOS select transistor and the source, drain and N well of the PMOS breakdown transistor are connected. The gate of the PMOS breakdown transistor is connected to the source of the PMOS program transistor.
The NMOS select transistor provides a first program voltage pulse to the N well of the PMOS transistor of a selected memory cell. The PMOS program transistor provides a second program voltage pulse to the N well of the PMOS transistor of the selected memory cell. The magnitude of the two combined voltage pulses is sufficient to break the twenty Angstrom (20 Å) thickness of the gate oxide layer of the PMOS transistor. Because the memory state of the memory cell depends on the breakdown status of the PMOS breakdown transistor, the data may be retained in the memory cell for an unlimited period of time.
Before undertaking the Detailed Description of the Invention below, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation; the term “or,” is inclusive, meaning and/or; the phrases “associated with” and “associated therewith,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like.
Definitions for certain words and phrases are provided throughout this patent document, those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior uses, as well as to future uses, of such defined words and phrases.